10:45 AM - 12:15 PM
[SSS10-P01] Seismicity analysis in and around the focal regions of the 2021 M7.3 and 2022 M7.4 earthquakes offshore Fukushima Prefecture, Japan
Keywords:b-vaule, p-vaule, Z-value, Stress state, Intra-slab earthquake
Spatiotemporal evolutions of seismic activity are presented for a broad region surrounding the focal areas of the 2021 M7.3 and 2022 M7.4 earthquakes, which occurred within the subducting slab off the Pacific coast of the Fukushima prefecture, northeastern Japan. This study investigates spatiotemporal distributions in seismicity parameters during periods before the 2021 M7.3 earthquake, between the 2021 M7.3 and 2022 M7.4 earthquakes, and after the 2022 M7.4 earthquake, using the b-value of the Gutenberg-Richter relation, the p-value for aftershocks, and the seismic quiescence z-value. Focused on the depth variations relative to the plate interface, b-values for the deeper side (= approximately intraslab side) are generally lower than those for the shallower side (= approximately plate interface side) throughout the entire analysis period, including the hypocentral areas of the M7.3 and M7.4 earthquakes. Aftershock decay rates for M7.3 and M7.4 earthquakes also show depth-dependent characteristics, with a slower decay rate for the deeper side than the shallower side. Furthermore, about two years before the event, a seismic quiescence was found in the hypocentral area of the 2021 M7.3 earthquake. The M7.3 and M7.4 earthquakes are located around the down-dip of the 2011 Tohoku earthquake with M9.0. Seismic anomalies detected in this study are mainly considered to reflect a stress increase due to the coseismic and postseismic slips of the Tohoku earthquake. The present study suggests that the effect of viscoelastic relaxation is a dominant factor for the deeper side. In addition, heterogeneous structures within the slab may reflect the detected seismicity anomalies.